Expanding the active charge carriers of polymer electrolytes in lithium-based batteries using an anion-hosting cathode

Ionic-conductive polymers are appealing electrolyte materials for solid-state lithium-based batteries. However, these polymers are detrimentally affected by the electrochemically-inactive anion migration that limits the ionic conductivity and accelerates cell failure. To circumvent this issue, we propose the use of polyvinyl ferrocene (PVF) as positive electrode active material. The PVF acts as an anion-acceptor during redox processes, thus simultaneously setting anions and lithium ions as effective charge carriers. We report the testing of various Li||PVF lab-scale cells using polyethylene oxide (PEO) matrix and Li-containing salts with different anions. Interestingly, the cells using the PEO-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solid electrolyte deliver an initial capacity of 108 mAh g−1 at 100 μA cm−2 and 60 °C, and a discharge capacity retention of 70% (i.e., 70 mAh g−1) after 2800 cycles at 300 μA cm−2 and 60 °C. The Li|PEO-LiTFSI|PVF cells tested at 50 μA cm−2 and 30 °C can also deliver an initial discharge capacity of around 98 mAh g−1 with an electrolyte ionic conductivity in the order of 10−5 S cm−1.

dendrites in SPEs. Therefore, our discussion of dendrite's forming mechanisms in the original manuscript has been revised, and more details have been added. In the revised version, we refer to the works about multi-systems previous reported and modify the discussion in this part.
The polarization results in critical performance degradation, such as increasing internal resistance and decreasing operating voltage 27 . Coupling with the low Li + migration increasing the electrode/electrolyte interface resistances significantly to accelerate the deterioration of anodes [28][29][30] . To verify that minimizing the concentration polarization could effectively enhance the lithium anode stability, we tested LiFePO 4 |PEO-LiTFSI|Li batteries under the same condition (Fig. S20). As shown in Fig. 5c, 5e, a micro short circuit occurred in LiFePO 4 -2 . The short circuit has not been repaired in subsequent cycles, leading to a continuous decayed in coulombic efficiency (Fig. S20b).
Short circuits in cell with solid polymer electrolytes are extensively elaborated in recent works and should be not ignored in this passage: http://dx.doi.org/10.1039/D1MA00009H https://doi.org/10.1038/s41598-020-61373-9 Response: The reviewer's literature suggestion for this point is very reasonable. In addition, there have been related in-depth studies on the short circuit phenomenon of SPE-based solid-state batteries. Thus, we refer to the recommended studies and review the latest related work. Additional discussion and citations have been added to the revised version.
A similar degradation mechanism has been intensively studied in recent works. It was confirmed that high current density and lithium metal deposition accelerates the initial micro short circuit phenomenon, even in the first few cycles 56,57 . The former literature shows that a constant distance (realized by a simple spacer) can prevent short circuits originating from dendrites as the spacer prevents the shrinkage in electrode distance, thus the risk of short circuits. My suggestion in this regard (maybe for future work) is to validate these cells with such a spacer, as this would reveal more systematic data (constant distance between the electrodes), as otherwise the PEO is mechanically prone to shrinkage.

Response:
Thank you very much for the valuable suggestions of the reviewers. In the main part of this work, the carriers' expansion approaches allowed the polymer electrolyte to sustain more than 4000 cycles without a short circuit. Most of the similar electrolytes in the literature could not last more than 30 cycles. In addition, we have added to the revised manuscript the results of high mass loading tests (Fig. S30, S32).
Due to the large volumetric changes at high loading, more pronounced deformation is taking place in the anode and cathode during the cycles. This leads to the failure of the batteries with LFP cathodes after a few cycles, while our designed cathode with PVF addition could survive cycling for more than 100 cycles at such a high salt loading. However, we also agree with the reviewer's suggestion that controlling the anode-cathode spacing can limit the growth of dendrites and believe that this strategy will be of great help for mixed or high-load electrodes in future research.

Reviewer #2 (Remarks to the Author):
anionpositive active material in Li-metal polymer battery. Although some results are promising, the article needs a major revision before to be considered for publication in high impact journal like Nature Communications. Especially I have some concerns about cathode preparation and the loading used.

Response:
We thank the reviewer for the careful review of our manuscript and the insightful comments. We respond to each question of the reviewer one by one below. We have considered all the reviewer's comments and modified our paper accordingly. Overall, the paper has been subjected to a major revision with lots of discussion, experimental results and simulation results have been added to support our claims. The revised texts in the manuscript and supporting information are highlighted in yellow. Please find the details below.

Response:
We are very grateful to the reviewers for their suggestions. We have added the recommended references and comments of related work in the revised edition. The introduction part now provides a more comprehensive review of the reported work.
The safety requirement for lithium-ion batteries and the demand for lithium metal anode has prompted researchers to look for solid-state alternatives to liquid organic electrolytes 1-4 .
Typical strategies include designing the polymer segment structure 10-12 and combining polymer matrix with ceramic 13,14 or inorganic solid electrolytes 15,16 .
The facile movement of the anions and the scarce supply of cations facilitates the uneven deposition of Li onto surface protrusions at high current density, leading to the self-amplification process of dendritic growth 22-24 . 2) The authors must add a SEM image and EDS mapping of the cathode and include the results in Figure 2.

Response:
The reviewer's suggestion on this issue is very reasonable. We agree that electrode morphology and element distribution is important. Therefore, the SEM and EDS mapping of the electrode at different magnifications. The results prove that the active and conductive materials are evenly mixed without significant agglomeration.
We agree with the reviewer's concern about electrode mass loading and have performed new experiments with different mass loading. The current carrying capacity of solid electrolytes (especially SPEs) is much lower than that of liquid electrolytes, which causes the load capacity of the electrode to be incomparable with liquid-based batteries and usually cannot achieve high areal capacity. On the other hand, the PVF used in this work is a typical organic material, and the inherent low ion/electronic conductivity requires a large amount of conductive agent for sufficient electrochemical reactions even in liquid systems (Energy Environ. Sci. 2013, 6, 2280, Nat. Rev. Chem. 2020. Therefore, the high loading is a massive challenge for PVF electrodes.

Response:
We are very grateful for the suggestions made by the reviewers, which we found very helpful to improve the quality of our paper. Based on the reviewer's suggestion, we have modfield the discussion about the Li + movement mechanism in the revision.
Yet, the weak polymer chain motion leads to insufficient ion transport for SPEs even at high temperature 3,9 .
Specifically, the PEO chains can adopt a helical conformation that presents the optimum distances for O-Li interactions similar to the crown structures 18,19 . 6) What is thickness of lithium metal? High energy dense lithium metal battery requires thin or free lithium. Considering the high homogenous lithium plating reported in the manuscript, the authors should prepare a battery with a lithium less configuration cell (direct plating on copper foil).

Response:
Thanks for the suggestion raised by the reviewer. The thickness of lithium foil is . We agree with the reviewer's view that high energy density batteries require less lithium or no lithium electrodes. Mortified by the excellent anode lithium stability observed in the current, we have carried out tests on lithium-free anode batteries. However, the performance was not so promising. The results indicated some reaction between the solid electrolyte and the anode, despite the stable deposition of a) Materials Science and Engineering: R: Reports 134, 2018, 1-21; b) Angew.Chem. Int. Ed. 2019, 58,15978-16000

Response:
We thank the reviewer for this suggestion. As the reviewer stated, the importance of fluorine in liquid batteries is widely investigated (Adv. Mater. 2018, 30, 1706375, J. Am. Chem. Soc. 2017, 139, 11550). The fluorine-containing electrolyte components, including salts (e.g. LiTFSI, LiFSI) and solvent additives (e.g. FEC), are beneficial to promote LiF formation on the anode side. However, the main factor of the degradation of SPEs-based batteries (especially rate performance) is the ionic conductivity. In this work, since the polymer matrix is PEO, the salts (anions species) directly affect the complexing of ions. LiTFSI is able to dissociate easier than LiFSI, giving higher Li + conductivities in the SPEs. This discussion has been added to the revised manuscript, and the suggested literature has also been cited.
Since the migration of anions and cations in the present work is related to the n 43,44 , we set several types of lithium salt in SPEs to obtain a deep insight into the anion electrode reaction.

Response:
We are very grateful to the reviewer for this suggestion. The approach that we introduce in the current paper would enable to use PVF as a modifier for several electrodes as suggested by the reviewer. The classic polymer matrix (PEO) used in this work does not match the high voltage cathode (NMC) well. However, it is reasonable to speculate that the subsequent experimental results can be extended to electr 4815 g mol -1 ), and its distribution is wide (Mw/Mn = 1.708). Therefore, some of the fragments with small molecular weight might diffuse to the solid electrolytes at the test temperature (60 °C), reducing the active electrode materials.
In the case of high mass loading, the organic molecules also tend to dissolve into the same electrolytes, resulting in a poor cycle life (Materials 2019, 12, 1770).
However, this issue could be solved in the follow-up research to determine the optimal composition. Furthermore, designing and constructing high-efficiency host materials makes it possible to achieve physical/chemical adsorption of molecules, thereby avoiding a significant decrease in capacity. Therefore, we believe these strategies could be valuable in using polymers as active materials in solid-state batteries. Am. Chem. Soc. 2017, 139, 1207. Compared with ferrocene, other metallocenes (Co or Ni as metals atom) are uncommon and much expensive (metal elements). The higher molecular weight reduced the capacity. Therefore, the ferrocene-based polymer is more advantageous than other metallocenes in preparing anion hosting cathode.

Reviewer #3 (Remarks to the Author):
Authors investigate a solid-state dual-ion battery using the typical PEO as polymer matrix and a redox-active polymer as cathode. This system is introduced to avoid the concentration polarization observed with SPEs and provide an active use of the anions, which is significant to the field.
The whole paper is focused on the fact that the improved electrochemical performance of PVF|PEO:LiTFSI|Li is due to the avoidance of concentration gradients. However, there is not enough proof to confirm the hypothesis and therefore further experiments and explanations are needed. This is justified in the following specific as well as general comments:

Response:
We thank the reviewer for the valuable comments. To support the hypothesis/proposed mechanism, we have added several related pieces of literature and provided additional experimental data to the revised version. In addition, we have re-organized our discussion and strengthened our claims, trying our best to address the comments and suggestions fully. Below please find our point-by-point responses.
1. I suggest the authors to mention that this is a dual-ion battery as it is an already in use term to describe this type of battery where anions take place in the reaction. This might help the reader understand the topic.

Response:
Thanks to the reviewers for this suggestion. The description of the battery system is critical, and we very much agree with the reviewer's definition of the dual-ion battery to the system presented in our paper. The original intention of this work is to solve some of the challenges of SPEs based solid-state batteries due to the lack of effective carriers. To distinguish it from the reported dual-ion gel electrolyte, which focuses on liquid dual-ion batteries' problems, we use the "expand effective carrier" description. We believe this term will be less confusing to the reader. Corresponding changes have been made in the revised version.
The participation of anions in electrode reaction has promoted the development of dual-ion batteries in the liquid or extended gel phase 36-38 .
the electrolyte of dual-ion batteries is similar to that of lithium-ion batteries.
In this work, to solve the problem of SPEs due to the non-reactive anion migration in SPEs, we build dual-ion solid-state batteries through anion-hosting cathode.

Response:
Thanks to the reviewers for their suggestions, which allows us to improve the manuscript. Lithium-ion is the counter ion of polyanion based single-ion solid polymer electrolytes (SISPEs). However, the strong electrostatic interaction causes the lithium ions to combine with negative charges and render it inactive in the electrochemical reaction, or in other work, the quantity of free Li + is small. This is distinguished from the low number of lithium ions caused by the weak dissociation ability of ion clusters and ions in the polymer-salt system. We made relevant clarifications in the revision.
Though some studies have proved that SISPEs have less strict requirements for ion conductivity 32,35 , there are still concerns about the number of effective carriers due to strong electrostatic interaction between Li + and the negative charges.
3. Page 3 line 55, authors mention that the cost of using SISPEs is the low ionic conductivity due to the lack of solvation ability. The ionic conductivity of SISPEs is considered low compared to conventional SPE due to the fact that anions do not contribute to the ionic conductivity, so it is not a fair comparison. In addition, they ascribed it to the lack of solvation ability. The solvation ability is given by the polymer matrix either if it is in a conventional SPE or SISPEs. Further clarification would be beneficial.

Response:
Thank you for the precious suggestion. We agree with the reviewers' views on comparing ion conductivity between SPEs and SISPEs. In the first submission, we did not emphasize that the relatively high ionic conductivity in traditional SPEs cannot fully participate in the electrochemical reaction. In addition, the description of the

Response:
We totally agree with the reviewer suggestion. In Fig. S5b, we have added a partially enlarged view of the DSC curve of SPEs, showing the effect of different anion species on Tg. In general, the Tg of SPEs usually reflects the difference in ion mobility. However, in the current study, the link between the ions' mobility and Tg is has overall good performance (at least at high temperatures) confirmed by multiple articles and commercial batteries built with this system. I suggest to rephrase it.

Response:
We agree with the reviewer that the way it was written in the first submission is confusing. The original submission implies that the performance is mainly determined by t Li+ only, which is not true, as pointed out by the reviewer. While at low current density, t Li+ may be used to predict the cell performance, this is not the case at high current. The importance of t Li+ will be highlighted when the conductivity is insufficient. We modified the discussion in the revised version to make the statement clearer.
Generally, for a typical Li + -hosting cathode, SPE with low t Li+ has strict requirements on ion conductivity, especially under high current density, the scarcity of cations on the electrode surface accelerates the growth of dendrites 22 .

Response:
We agree with the reviewer that, in addition to the ionic conductivity, some other factors might affect the cell overpotential, as discussed in the literature with many batteries systems. The motivation to investigate the anion types is not only because of their influence on the cell overpotential but also their effect on the interaction process.
Regarding the influence on the cell overpotential, our experimental results confirmed that the difference in ionic conductivity caused by salts type affected the overpotential of batteries (Fig. S10), calculated from the difference between the redox anion type. proof of such statement. This section would benefit from further explanations and mentioning more clearly to which experimental results they are referring to. While the difference in experimental electrode potentials is large, the difference from the calculations is very small. In addition, what is the theoretical value? Is the anion the only contributor to the shift in voltage plateau? But before this difference in redox peaks and plateaus was assigned to the difference in ionic conductivity.

Response:
We are very grateful to the reviewers for raising this question, which allows us to provide additional relevant experimental results to discuss the issue further. To prove the influence of steric hindrance on the electrode potential (E 0 ) generated by the binding energy, we set the linear ferrocene structure (constructed through click-chemistry) as cathode, named VFS, which can avoid small molecules (ferrocene) diffusing and deactivating in the electrochemical process. At the same time, it has a significantly lower steric hindrance, lead the effect of binding energy more obvious.
The CV tests using VFS cathode showed that the electrode potential (E 0 ) with four different type anions is linked to binding energy, matching the computation results except for BOB -. The E 0 decrease of BOB --based reaction is not obvious, which has a specific deviation from binding energy. This result can be attributed to the steric effect of anion-dominated ion clusters. We investigated the BOB --based reaction at O: Li ratios of 30: 1 and 40: 1, the E 0 of the VFS cathode is significantly lower than that of the PVF cathode. The above results prove that the steric hindrance of large-volume anions (TFSI -, BOB -, FSI -) with polymer cathode and the ion clusters (BOBat 20:1) dominated ion form will both significantly impact the reaction

Response:
We are very grateful to the reviewers for their valuable suggestions. To fully confirm the hypothesis of this article, we performed tests of mixed cathodes and n-type polymer cathodes. The results well verified our theory: introducing anions to electrode reactions can lead to stable cycles. Regarding the controlled factors of the anion participated electrode reaction, by separately controlling the cathode (low steric active material) and the steric hindrance of the anion (ion cluster), we have a deeper understanding of the anion migration in this system. This part is also benefited from the related work about anion chemistry and analysis of our supplementary experiments.
Due to the better dissociation of lithium ions and stable anion structure, LiTFSI salt shows the broadest range of excellent results in solid-state batteries with traditional lithium-ion hosting cathodes. However, as for the case where cations and anions both participate, ionic conductivity is not the only factor affecting the electrode reaction. The excellent performance TFSIshowed has been deeply studied in this work. The conclusion that the cathode and anion influence also provide a reference for further work design and exploration.
As the earliest and most widely studied polymer electrolyte matrix, the research on the physical and chemical properties of PEO has been relatively mature. We chose it to verify the viewpoints of our design in the simplest electrolyte. We very much agree with the reviewer's research suggestions on other electrolyte systems, and we believe that the combination of hybrid cathodes and more advanced electrolytes can be demonstrated in future research work.

Reviewer #1 (Remarks to the Author):
A very good revision and general work. Should be accepted in present form.

Response:
We thank the reviewer for the positive comment and support.

Reviewer #2 (Remarks to the Author):
The authors have well revised the paper and made huge efforts to improve the quality of the paper. The paper is interesting and well written. I recommend a minor revision.

Response:
We thank the reviewer for the positive comment and support. We fully agree with the referee's concern about figure arrangement. In order to avoid irrelevant data in the article, we removed the Li foil) that cannot effectively reflect the stability of lithium metal. In response to the mass load issue raised by the referee, we have emphasized the performance of high loading electrodes and made additional discussion to the revision.

Page 8, line 213
Importantly, for the high mass loading tests, pure LFP electrode quickly developed a micro-short circuit during the initial few cycles (Fig. 5e, S30) and failed to work at a capacity around 1 mAh cm -2 . In contrast, mixed cathode (mass ratio, LFP: PVF = 1: 1) showed clear cycling improvement, maintaining more than 90 cycles without short circuits.

Reviewer #3 (Remarks to the Author):
The manuscript has improved after the revision; however, the effect of the anion is

Response:
We thank the reviewer for pointing out this problem to help us improve the article's reliability. As the referee said, the ionic conductivity does not correspond well with the DSC results. The polymer-ion aggregation forms led by anions are different, and the chain segment movement can not be directly linked to the ion motion. Ionic conductivity should result from the combined effects of the physical properties of the polymer (glass transition temperature, melting point, etc.) and the ionic structure within it. We have explained this in the revised version.

Page 5, line 95
The DSC results show that, relative to pure PEO, both SPEs exhibit decreased Tm and produce glass transition processes, implying enhanced segmental motion.
LiTFSI and LiBOB, which exhibit high ionic conductivity, have a lower degree of crystallinity (determined by the melting peak intensity). The Tm and Tg changes with anions species are not directly related to the ionic conductivity.
Different anions impact the polymer-salt composite structure, resulting in various degrees of segment motion. Meanwhile, salt dissociation affects the conductivity.
The negative charge delocalization of anion helps release more free ions.
Although SPEs are less mobile than liquids, however, above the melting point, the motion of the polymer chains still puts the electrode material at risk of diffusion.
Specifically, currently reported organic molecule-based solid-state batteries mainly adopt inorganic solid electrolytes (ISEs) to avoid cathode loss (Angew. Chem. Int. Ed., 2018, 57, 8567, ACS Energy Lett., 2021, 6, 201-207, Joule, 2019, 3, 1349-1359. Thus, as stated by referees, the diffusion of active material could be significant with liquid electrolytes but cannot be easily ignored in SPEs. 6 and fig 4: the focus is on the potential difference, but the shape of the CV is also different depending on the anion. Nothing about that is mentioned. Furthermore, in my opinion, the explanation of the E 0 , BE and anion could benefit from clearer explanations.

Response:
We thank the reviewers for their suggestions, which were significant in improving the article. As stated by the reviewer, the CV shape correlates with the anion species.
In the TFSI --involved CV, the primary redox process was followed by a recessive weak peak. However, the redox peaks with different shapes also have good reproducibility, which indicates that the process is also reversible. In liquid systems, CVs of ferrocene exhibit non-ideal behavior, commonly observed in close-packed Fc self-assembled monolayers (SAMs) (Langmuir, 2006, 22, 4438-4444, J. Phys. Chem. C., 2015, 119, 21978-21991, J. Phys. Chem. C, 2013, 117, 1006-1012. These can be attributed to reasons including local heterogeneity and intermolecular interactions. The close packing of ferrocene units is also involved in this work, and the splitting and shape difference of CV peaks should be related to this. Taking TFSIas an example, multiple redox waves (or so-called peak splitting) appear in the CV peak when PVF is used as the cathode. However, in the CV results of VFS cathodes where the ferrocene units are relatively dispersed, there is no apparent peak separation, which fully proves that the aggregation morphology of the active cathode units has a significant effect on the CV shape. For the anions that are bulky or have special strongly electronegative atoms (F atoms), the buried active unit (ferrocene) can lead theoretically potential origins for the anion-related properties of peak shape, including: (1) Buried Fc and plane of electron transfer (PET). Active unit (ferrocene) packing in PVF buries part of the Fc. As described in the previous experiments and model about close-packed Fc self-assembled monolayers (SAMs) (Anal. Chem., 1992, 64, 2398-2405, J. Phys. Chem. C, 2015, the positional difference between the active unit and the closest ion/ion cluster creates multiple electron transfer planes (PETs) that induce splitting and broadening of the CV peak. We noticed that making the units farther apart ((VFS cathode with TFSIanion, shown in Fig. S11) obtained a more ideal CV.
(2) Intermolecular interactions experienced by the Fc and anions. For Fc surface-bound redox activity, CVs can be fitted to models based on Langmuir or Frumkin isotherms to gain insight into the nature of intermolecular interactions (J.
In addition, we thank for the suggestion about the explanation, and revised manuscript to make the information clearer.

Page 6, line 137
The results show that the ion pairs' formation could negative shift the electrode potential (E 0 ) from theoretical, affected by binding capability 52,53 . We calculate the binding energy (BE) of ion pairs by density functional theory (DFT) simulations.
The cathode was simplified by substituting ethyl ferrocenium for the PVF (Table   S4). As Fig. 4a shows, the ethyl ferrocenium has the highest BE to ClO 4 -, and decrease with the order FSI -, BOB -, and TFSI -. However, in the CV results of PVF, the E 0 with TFSI -, BOB -, and FSIas anions have no significantly differences (3.463, 3.470, and 3.476 V, respectively). The discrepancy between the experimental and computational results can be explained by the steric hindrance of active units (ferrocene) and ions/ion clusters: (1) The folded long chain in PVF inhibits the binding of large anion to ferrocene, thus, reducing the BE effect on E 0 .
When using VFS (Fig. S11a), with low steric hindrance for ferrocene, as cathode, the E 0 of TFSI -, FSIdecreased significantly (Fig. S12a), matching the trend of the calculated results; (2) The anion-dominated ion clusters exhibit a larger steric structure, which enhances the hindrance effect. In PEO-LiBOB, the change in salt concentration resulted in different aggregation morphologies of ions, as previously described (Fig. 3b, S7). Compared with O: Li at 20: 1, the decrease of E 0 (PVF to VFS) is more evident in low salt concentration (30: 1, 40: 1), shown in Fig. S11c, S12b. The results demonstrate that, apart from the cathode, the steric hindrance on anion side also weakens the E 0 drop caused by the binding process. Therefore, avoiding ion clusters, ClO 4 -, with the smallest size ( Fig. S13) and the strongest BE ( Fig. 4c), shifts E 0 to more negative values (3.381 V vs. Li + /Li), while the other anions were not significantly affected by the binding effect (Fig. 4d).
6. Temperatures of every experiments should be clearly marked in the text and figure captions.